1. Field of the Invention
The present invention relates to liquid dispensers and printers. The present invention particularly relates to a liquid dispenser including heating elements arranged to be adjacent to each other and a strip protective layer, having slits disposed between the heating elements, for covering such heating elements and also relates to an inkjet printer. In the protective layer, cracks are securely prevented from being caused.
2. Description of the Related Art
In recent years, needs for colored hard copies have been increasing in the field of image processing and so on. In response to such needs, the following color-copying systems have been conventionally proposed: a sublimation-dye transfer printing system, a thermofusible transfer system, an inkjet system, an electrophotographic system, and a thermal development system.
In the inkjet system, which is one of the above-mentioned systems, droplets of recording liquid (ink) are ejected from a nozzle provided in a recording head, which is a liquid dispenser, so as to form dots on an recording object, whereby a high-quality image can be output with a simple configuration. The inkjet system is classified into an electrostatic attraction method, a continuous oscillation generating method (a piezoelectric method), a thermal method, and so on, depending on the difference of methods for ejecting ink.
In the thermal method, which is one of the above-mentioned methods, bubbles are generated by locally heating ink and ink droplets are then pushed out from nozzles by the bubbles such that the ink droplets are applied to a printing object, whereby the printing of a color image is possible with a simple configuration.
A printer using the thermal method includes a so-called printer head. The printer head includes a semiconductor substrate, heating elements for heating ink, a driving circuit, which is of a logic integrated circuit type, for energizing the heating elements, and so on, wherein these components are disposed on the semiconductor substrate. Thereby, the heating elements can be densely arranged and securely energized.
In the thermal printer, in order to obtain high-quality printouts, the heating elements must be densely arranged in the printer head. In particular, in order to obtain, for example, 600 dpi printouts, the heating elements must be arranged at an interval of 42.333 μm. However, it is difficult to provide driving elements to the corresponding heating elements that are densely arranged. Therefore, the printer head further includes switching transistors that are formed on the semiconductor substrate and connected to the corresponding heating elements using integrated circuit techniques. The driving circuit, also disposed on the semiconductor substrate, drives the switching transistors to securely energize the corresponding heating elements in a simple manner.
In the printer head, the heating elements are energized to generate bubbles, ink droplets are ejected from nozzles by the bubbles, and the bubbles in a liquid chamber then disappear. Thus, the generation and disappearance of bubbles are repeated at a short time interval of several μseconds, which corresponds to the cycle time of the ejection of the ink droplets. The heating elements are adversely affected from mechanical shock caused by cavitation arising during the repetition.
Therefore, in order to protect the heating elements, the printer head further includes an insulating layer and an anti-cavitation layer on the heating elements. As shown in FIG. 4, a conventional printer head 1 similar to the above printer head includes a semiconductor substrate 2, semiconductor elements, first heating elements 3, a first insulating layer 4, wiring lines 5 for connecting the first heating elements 3 to the corresponding semiconductor elements, a second insulating layer 6, and a first anti-cavitation layer 7 functioning as a protective layer. These portions are formed according to the following procedure: a resistive layer comprising a resistive material such as tantalum, tantalum nitride, or tantalum-aluminum alloy is formed on the semiconductor substrate 2 by a sputtering method; the resistive layer is etched into the first heating elements 3; the first insulating layer 4 comprising silicon nitride or the like is formed on the first heating elements 3 by a deposition method; a layer comprising, for example, aluminum is formed on the first insulating layer 4 and then patterned to form the wiring lines 5; the second insulating layer 6 comprising silicon nitride or the like is formed on the wiring lines 5 by a deposition method; and the first anti-cavitation layer 7 comprising an inorganic material such as tantalum is then formed on the second insulating layer 6. In the conventional printer head 1 having the above configuration, the first heating element 3 has high heat resistance and superior insulating properties and is prevented from making direct contact with ink droplets, and the mechanical shock caused by the above cavitation is lowered to protect the first heating element 3.
The following techniques are disclosed in Japanese Examined Patent Application Publication No. 5-26657: a conventional technique in which anti-cavitation layers are each independently provided to corresponding heating elements and a new technique in which a strip anti-cavitation layer is provided so as to cover a plurality of heating elements.
In general, when an insulating layer and/or an anti-cavitation layer of a printer head have a small thickness, ink droplets can be ejected with a small amount of electric power because heat generated by heating elements can be effectively transmitted to ink.
However, when the thickness of the above layers is reduced, the reliability of the printer head is also lowered. That is, when the insulating layer comprising silicon nitride or the like has a small thickness, pinholes are readily caused in the insulating layer and poor step coverage is caused at regions of the insulating layer covering steps of wiring lines. Therefore, when the thickness is too small, ink penetrates the printer head through the pinholes and the regions having poor step coverage to corrode wiring lines and heating elements, thereby causing breaks therein.
Therefore, in the printer head, the insulating layer and the anti-cavitation layer must have a thickness sufficient to prevent such pinholes and poor step coverage from arising.
In the printer head, since the heating elements are repeatedly heated at a short time interval of several μseconds, which corresponds to the cycle time of the ejection of the ink droplets, a large heat stress is repeatedly applied to the insulating layer and the anti-cavitation layer. Thus, there is a problem in that the reliability of the printer head is lowered due to the penetration of ink even if the insulating layer and the anti-cavitation layer have a thickness sufficient to prevent the pinholes and poor step coverage from arising.
In particular, as disclosed in Japanese Examined Patent Application Publication No. 5-26657 described above, when the anti-cavitation layer has a strip shape so as to cover a plurality of the heating elements, cracks are readily caused and therefore the reliability is significantly lowered because stress is concentrated on one portion of the anti-cavitation layer.
The anti-cavitation layer comprising tantalum has a large compressive stress of 1.5×e10 to 2×e10 dynes/cm2. According to an experiment, when the tantalum anti-cavitation layer is laid in a 400° C. atmosphere for 60 minutes, cracks are caused in the insulating layer comprising silicon nitride. A region where a crack is caused is shown in FIG. 4. When such a crack is caused, ink penetrates the printer head through the crack to corrode the wiring lines and the heating elements, thereby causing breaks therein.
In order to solve this problem, the following technique disclosed in the Hewlett-Packard Journal, May 1985, pp. 27-32 can be used: wiring lines are processed by a wet etching method so as to have a round corner, and end faces of the wiring lines are tapered, thereby heightening the step coverage at regions covering steps of wiring lines and thereby preventing stress concentration. This technique is effective when the wiring lines comprise only aluminum. However, in actual practice, the wiring lines comprise aluminum alloy containing silicon, copper, and the like in order to improve the characteristics thereof. Thus, when the wiring lines comprising such alloy are used, residues are formed to cause dust, which is harmful to a semiconductor manufacturing process. Accordingly, there is a problem in that this technique cannot be used for the above printer head.